Control systems



Dec. 16, 1958 'r. w. JENKINS, JR

CONTROL SYSTEMS 2 Sheets-Sheet L Filed Jan. 27, 1953 Dec. 16, 1958 VT.w. JENKINS, JR

CONTROL SYSTEMS 2 Sheets-Sheet 2 Filed Jan. 27, 1953 3% m S m MT 7 8 M 3h m Mm 4 l. P 2 3 4 5 f I l O b 9 m n i m M 5 I m 4 mm 2% m. 4- AT 5 F 61 l M S 5 l mm 5 .m NB m 5 P I: 8 5

Temperature CONTRGL SYSTEMS Theron W. Jenkins, .lr., Fort Washington,Pa, assignor to Leeds and Northrop Company, Philadelphia, 3%., acorporation of Pennsylvania Application January 27, 1953, Serial No.333,494

9 Claims. (Cl. l22479) This invention relates to control systems and hasfor an object the provision of a system in which each of themagnitudesof two conditions which normally vary in dependentrelationship may be regulated without substantial change of the othermagnitude.

The present invention is particularly applicable to power generatingsystems of the type which include turbine-generators in which the steamafter passing through one group of stages of the turbine is reheated andthen passed through a second group of stages of the turbine. The steam,generally elevated to high temperatures and pressures, is dischargedfrom superheater coils to the first section of the turbine. Indeveloping a part of the power needed to turn the generator there is inthe turbine a decrease in the temperature and pressure of the steam. Ithas been found that higher operating efliciencies may be attained byagain heating the steam as by passing it through a reheater section ofthe boiler to elevate again its temperature and then passing the steamsuccessively through the several stages usually present in a steamturbine.

In the generation of power, the trend has been to utilize hightemperatures and high pressures to attain high efficiency. In addition,the maintenance of a constant temperature results in eliminating thermalexpansion and contraction of the turbine compoents. However, as thetemperatures and pressures have been increased, so has the importance ofa control system which will insure operation below the danger point asregards the strength of the tubes and lines through which the steampasses. The nearer the temperature approaches that of the yield point ofthe tubes, the more important it is that the control system prevent riseof temperature into the danger zone. Accordingly, in furnaces in whichsteam superheater coils and steam reheater coils are both disposed inheat exchange with hot products of combustion from the fire chamber, itis apparent that there must either be provided separate control of thetemperature of the steam in the outlet of each of the respective coilsor one of them must be operated in manner such that its temperature cannever rise into the danger zone. The latter alternative would beinefiicient, and yet when two coils or heat exchange sections arelocated in heat exchange with hot products of combustion a major problemof control is presented since a change in the heating relationship asmay be required to maintain the outlet temperature of the steam in onecoil at a desired value will, of course, change the outlet temperatureof the steam from the other coil.

It is a further object of the present invention to provide a controlsystem for a steam generator of the foregoing type in which there areindependently controlled the outlet temperatures from coils disposed inheatexchange relation to the hot products of combustion.

In accordance with the present invention, it has been found that wherethe system to be controlled embodies two temperature orcondition-varying elements where one of them affects both of theconditions to be regulated Patented Dec. 16, 1958 ice in the same sense,and the other affects both of the conditions, one being affected in asense opposite to that of the other, the magnitudes of the twoconditions may be maintained at selected temperatures each of which maybe changed independently of the other but both maintained at the newlyselected temperatures which establish the control points.

As applied to a boiler or steam generator with superheater coils in onesection receiving hot gases and products of combustion from the firechamber and with a reheater coil in another section likewise receivingthe products of combustion from the fire chamber, one condition-varyingelement comprising means for recirculating hot products of combustionwill affect both steam temperatures in the same sense, while the secondconditionvarying clement, namely a damper control for regulating thedivision of hot products of combustion as-between the superheater coilsand the reheater coils, will affect both steam temperatures in anopposite sense.

Further in accordance with the invention, one condition-varying element,such as the output of the recirculator for the hot combustion productsunder the control of a damper, is placed under the control of meansoperable in response to changes in the algebraic sum of the magnitudesof the two conditions, i. e., the respective steam temperatures. Thecontrol means, of course, operates the condition-varying element in adirection and by an amount to maintain the algebraic sum atapproximately a predetermined value. There is also provided a secondcontrol means operable in response to difference between the magnitudesof the two conditions, again therespective steam temperatures, foroperating the second condition-varying element (the damper control) in adirection and by an amount to maintain a desired relationship betweenthe magnitudes of the two conditions. Thus, in one application of theinvention, there is provide-d independent control of the temperature atthe outlet of the superheater section and the temperature at the outletof the reheater section.

It is to be understood that the present invention is of generalapplication to systems in which there are present .a condition-varyingelement affecting two conditions in the same sense and a secondcondition-varying element affecting them in opposite sense.

For further objects and advantages of the invention and for a moredetailed description, reference is to be had to the followingdescription taken in conjunction with the accompanying drawings, inwhich:

Fig. 1 diagrammaticallyillustrates the invention embodied in anelectrical control system for a steam boiler;

Fig. 2 illustrates the invention as embodied in a pneumatic controlsystem for a steam boiler;

Fig. 3 diagrammatically illustrates the structure used for selecting orpredetermining the control point; and

Fig. 4 is a graph explanatory of the operation of one embodiment of theinvention.

Referring to the drawings, the invention in Fig. 1 has been illustratedas applied to a steam boiler 10 in which hot combustion gases from afire chamber 11, shown in Fig. 2 with suitable arrays of fuel burners 12and 13, flow through a passage 15 above a bridge wall 14. From thepassage 15 the gases, Fig. 1, pass downwardly over a secondarysuperheater 16. Steam is delivered to superheater 16 from a dividedprimary superheater including sections 17a and 17b. Steam fiow is fromthe steam drum (not shown) as by one or more pipes 18 through the twoprimary superheater sections 17a and 17b, the secondary superheatersection 16 and by way of a pipe 19 to the high-pressure section 20 of asteam turbine 21. The steam with reduced temperature is discharged fromthe high-pressure section 20 and flows by way of pipe 23 to the reheatersection 24 of furnace 10. After elevation of the temperature of thesteam it is discharged by way of pipe into the second section 22 of theturbine 21. Additional sections (not shown) for the turbine willordinarily be provided.

It will be observed that the secondary superheater section 16 is exposedto all of the hot products of combustion flowing through the passage 15and downwardly to a passage 26 which leads to a stack (not shown). Thehot gases and products of combustion then divide between the separatecompartments or passages in which the steam heating sections 17a, 17band 24 are respectively located. These separate compartments are formedby heat refractory bafiles 27 and 28, and the division of the gases andhot products of combustion is under the control of suitableflow-controlling elements shown as dampers 31, 32 and 33. The dampersmay be operated in any suitable manner. 7

When the condition-regulating means in the form of dampers 31 and 32 aremoved in a passa'gefclosing direction a g'reaterproportion' of thecombustion gases flows through the central passage, and when the dampers31 and 32 are moved in the opposite direction a greater proportion ofthe combustion gases will flow through the respective passages and lessthrough the central passage. As shown in Fig. 1, the division of thegases between the three sections is regulated with substantially uniformpressure drop as between passages 15 and 26 by reason of the fact thatwhen a control lever 34 is rotated, as by links 35 and 36 under thecontrol of a motor 37, it moves dampers 31 and32 in a passageclosingdirection and it moves damper 33 in a passageopening direction.

From the above description it will be seen that the outlet temperatureof the steam from sections 17a, 17b and 16, as measured for example by athermocouple 41, as well as the outlet temperature of the steam fromsection 24, as measured in pipe 25 by a thermocouple 42, will changewith a change in volume of products of combustion flowing from passage15 to passage 26. Those temperatures (other conditions remainingconstant) will also change with change intemperature of the hot productsof combustion. The relative temperatures of the outlet steam in pipes 19and 25 may be regulated by changing the division of the hot products ofcombustion as between the central passage and the t outside or flankingpassages.

In accordance with the present invention, it has been found that theoutlet temperatures, as in pipes 19 and 25, may be maintained withinreasonably close limits at selected values by regulating the position ofthe dampers to vary the division of the hot gases as between sections17a, 17b and section 24-, and by regulating the heat content ofcombustion gases flowing from passage 15 to passage 26 by suitablecontrol of a reci culating means. ing sections 17a, 17b and 24 per unittime, may be varied in any suitable manner, as by regulating theposition of a flow-controller or damper disposed in a passage 46 formingthe inlet to recirculating. fan 47 connected to a discharge pipe 48 forreturn of flue gases to the the chamber 11, as shown in Fig. 2. Whilethe speed of the recirculating fan may be varied to control the volumeof recirculated gases, normally the speed is constant and thepositioning of damper 45 is used to change or control to the desireddegree the regulation of the amount of recirculated gases and thus theheat content of the flue gases which flow from passage 15 to passage 26.

The manner in which the total heat contentof gases entering the heatingsections by way of passage 15 is regulated in accordance with thealgebraic sum of the steam temperatures as measured by thermocouples 41and 42, and the manner in which the division of the gases as between theseveral passages is determined by;

The heat content of gases flowing through heat- 4 the difference betweensaid two temperatures, will now be explained in detail.

The temperature of the steam in outlet pipe 19, as detected by thethermocouple 41, may be measured in any suitable manner as by apotentiometric network 50 including a battery 51, a variable resistor52, fixed resisters 53 and 54 and a slidewire resistor 55. By reason ofrelative movement provided between a contact 55a and slidewire resistor55 the voltage developed by the thermocouple 41 is opposed by a voltageor potential difference developed across a fraction of the slidewireresistor 55. The relative adjustment to produce null balance may beeffected manually or automatically as by the mechanical relay system ofSquibb Patent No. 1,935,732, or by a high-speed electronic balancingsystem, such as described in Williams Patents Nos. 2,113,164 and2,367,746.

As shown in Fig. l a difference or error voltage in the measuringcircuit is applied to an amplifier 56 which controls one winding 57a ofa motor 57, while the other winding 57b is energized from any suitablesource of alternating-current supply. The motor, in accordance with theoutput of amplifier 56, is energized to drive, through suitablemechanical connections indicated by the broken line 58, the contact 55ato reduce to zero the unbalance or error voltage in the measuringcircuit. At the same time it adjusts the position of a pen-index 59relative to a scale 60 and a chart 61 driven at uniform speed by anysuitable means such as by a synchronous motor 62.

In the same manner the temperature as indicated by thermocouple 42 isapplied to a measuring circuit including potentiometric network 63having a slidewire resistor 64 and a relatively adjustable contact 64a.The measuring system includes an amplifier 65 which controls theenergization of one of the motor windings of motor 66, the other motorwinding being connected to a suitable source of alternating-currentsupply, as illustrated. The temperature of the steam in line 25 isindicated by the pen-index 67 on scale 68 and is recorded on chart 69.

Control of the recirculating means by the positioning of damper 45 inaccordance with the sum of the two temperatures detected bythermocouples 41 and 42 is achieved by unbalancing a balanceable system,as by adjustment of circuit elements or variable impedances shown ascomprising contact 71a and its associated slidewire resistor 71relatively movable in accordance with change in position of pen-index59, a mechanical connection 73 illustrating the drive for contact 71a;and by relatively adjusting the contact 72a and slidewire resistor 72 asby mechanical connection 74 in accordance with the positioning of thepen-index 67. The slidewires 71 and 72 together with their shuntingresistors 75 and 76 are connected in a balanceable control network whichalso includes resistors 7780, a source of potential such as a battery81, and an adjustable rheostat 82. The output potential from the controlnetwork as developed by the contacts 71a and 72a is opposed by a voltageor potential dilference E developed by a control network including asource of supply, as battery 86, a slidewire resistor 87 with relativelyadjustable contact 87a driven by a motor 88. The control network alsoincludes a reset resistor 89, a reset capacitor 90, a pair of resistors91 and 92, a rate resistor 93, a rate capacitor 94 which may have inseries with it a resistor 95 In response to an unbalance voltage of onepolarity or the other between contacts 71a and 72a the motor 88 throughconventional means is energized for rotation in the forward or reversedirection to adjust, by way of mechanical connection 96, the damper 45and the contact 87a, the direction of rotation for a given polarity ofunbalance voltage being such as to return the sum of the twotemperatures 41 and 42 to a predetermined value.

As fully explained in Davis Letters Patent No.

2,666,170, dated January 12, 1954, and No 2,688,717, dated September 7,1954, the motor 88 is energized in direction and at a speed to maintainthe voltage or potential difierence E equal and opposite to thepotential difierence (E between slidewire contacts 71a and 72a.Accordingly, if the potential difference E be other than zero the motor38 will be energized to maintain movement of slidewire contact 87a forcontinued flow of current through resistor 89 and at least in part intoor out of reset capacitor Thus, reset action is introduced into theoperation of the control network 85. The voltage difference acrossresistor 39 is attenuated by rate resistor 93 when there is change inthe current flow through resistor 89 since under that condition currentwill flow by way of resistor 95 into or out of capacitor 94. By reasonof the attenuation, the rate of movement of contact 87a will be greaterwhen the rate of change of potential between contacts 71a and 72a ishigh than when it is low. Thus, the control network 85 includes a ratecontrol action. In addition to the rate and reset actions the controlsystem 85 also includes proportional action by reason of the follow-upadjustment of slidewire contact 87a by motor $8 in response to apotential difference between contacts 71a and 72a. The magnitude of theadjustment of contact 37a will include a component proportional to theextent of unbalance (the proportional action), a component due to thereset action, and a component due to the rate action.

It is to be understood the present invention is applicable to systems inwhich there is provided any desired combination of rate, reset andproportional action.

Assuming now that the sum of the temperatures as detected bythermocouples 41 and 42 exceeds a predetermined desired value, the motor88 adjusts damper 45 until the heat content of products of combustionflowing between passages 15 and 26 decreases by an amount which lowersboth temperatures to bring or to maintain their sum to its predeterminedvalue. When their sum decreases below the desired and predeterminedvalue, the converse action takes place and the damper 45 is adjusted toincrease the flow of combustion products recirculated by fan 17 toincrease said temperatures.

The desired temperature for the steam in line 19 as detected bythermocouple 41 may be selected by bodily moving slidewires 71 and 123relative to their associated contacts 71a and 123a. In practice, it isgenerally more convenient for the motor to drive the disk which carriesthe slidewire and to have the associated contact stationary. In thediagrammatic drawing of Fig. l, the slidewire contacts have been shownas movable and the same arrangement has been illustrated in thediagrammatic sketch of Fig. 3 in which the slidewires 71 and 123 areshown as supported by gears carried by a shaft 117 which is normallystationary but which through the operation of one or more gears 118a and1118b may be rotated as by a knob 11) to vary the position of eachslidewire relative to the contacts 71a and 123a which are rotated by themotor 57. The knob 119 is referred to as a control-point setter and itmay be arranged to drive an index 59a relative to the scale 60 to showin association with the pen-index 59 the predetermined temperature to bemaintained for the outlet steam in line 19. An arrangement such as shownin Ross et al. Patent No. 2,096,064 may be utilized, where the shaft ofFig. 2 of that patent would be driven by the motor 57 of accompanyingFigs. 1 and 3 where the disks 25 of the patent correspond with the gearscarrying the siidewires 71 and 123. The control-point setter or manuallyoperating element of the patent would then relatively adjust theslidewire positions with respect to shaft 10.

Returning to Fig. 1, it will be seen that if slidewires 71 and 123 arebodily moved relative to their associated contacts, the resultantunbalance voltages will respectively produce energization of motors 88and 37. If the control point temperature is lowered, the motor 88 willrotate in a direction to move damper 45 in passageclosing direction.Thus the quantity of hot combustion products passing in heat exchangewith sections 16, 17a, 17b and 24 will be decreased. However, motor 37will be energized in a direction to move dampers 31 and 32 inpassage-closing directions and dampers 33 in a passageopening direction.The effect will be to lower the temperature in line 19 but by increasingthe proportion of gas flow through section 24 there is maintainedtherein the same rate of heat absorption and thus the temperature of thesteam in line 25 is unchanged. In this way the temperature of the steamin line 19 will be lowered as determined by the control-point setterwithout change of the temperature of the steam in line 25.

In similar manner, a control-point setter of the same type as shown inFig. 3 is provided for predetermining the temperature of the steam inline 25. That controlpoint setter is associated with slidewires 72 and12 bodily to move them relative to their associated contacts 72:: and124a. The unbalance voltage in network 84 resulting from a selection ofa higher temperature produces energization of motor 88 to move damper 45in a passage-opening direction to increase the flow of hot combustionproducts while the unbalance voltage from network 121 energizes motor 37to move dampers 31 and 32 in passage-closing direction and dampers 33 ina passage-opening direction. Thus the rate of heat absorption in section24 is increased while the rate of heat absorption in sections 17a and17b is decreased by amounts adequate to maintain the same overall rateof heat absorption in section 16 plus that in sections 17a and 171;.Thus it willbe seen that any selected outlet temperatures for the steamin pipes 19 and 25 will be maintained by the system, the selected outputtemperatures then establishing the predetermined sum and differencetemperatures departure from which produces unbalance of the balanceablenetworks.

If desired, an additional control factor may be provided in mannergenerally set forth in said Davis Patent No. 2,688,717, the exampleshown in Fig. 1 including manometer lines or draft tubes 1111) and 191connected to a tilting U-tube manometer 1112 mounted upon a lever arm1113 supported for rotation by a pivot 1124-. By reason of thedifference in pressure detected by tubes 1% and 101, the fluid Withinthe U-tube, such as mercury, is caused to assume a difference inelevation in the two legs of the tube. Lever arm 1% moves core members105 and 106 relative to the primary windings 1 97 and 108 energized froma suitable source of alternatingcurrent supply. A secondary Winding 109applies to a full-wave rectifier 110 a voltage whose amplitude varieswith the position of core 105 between the coils 107 and 109. A fractionof the rectified output is applied by way of a slidewire 111 andconductors 112 and 113 to the control network and in series between themovaple contact 37a and the circuits including conductor 114 andresistor 89. Thus, the additional control action introduced into thenetwork 85 depends upon a change in value of the pressure difference,between the pressure at the inlet of the tube and that at the inlet ofthe tube 101.

By locating the open ends of the manometer tubes 1% and 101 in the draftpassage 115, Fig. 2, downstream or near the stack from the inlet to therecirculating fan 47, as on opposite sides of an air preheater oreconomizer section 116, the manometer tube 1112 will respond to the rateof flow of combustion gases. The flow rate, the total quantity of gasespassing to the stack per unit time, in turn depends upon the rate offuel combustion (assuming related proper adjustment of combustion air).A change in the amount or quantity of line gases will proide an actionwhich will anticipate the effects of such a change on the steamtemperatures. This will be obvious if it be assumed that the amount ofgases passing through the stack suddenly decreases. Such a decrease willmean each of, and the algebraic sum of, the temperatures of the steamflow in lines Hand 25 will decrease if no further corrective action beintroduced. However, the voltage output at the slidewire 111 resultingfrom operation of the manometer 102, introduces a potential differencein control network 85 which is effective to energize the motor 88 in adirection to open damper 45 for increased recirculation of combustiongases, the effect being to anticipate the expected decrease intemperature and thus to prevent its realization and'to diminish thedegree of temperature fall which would otherwise he realized.

By means of a balanceable system such as a control network 126 and apotentiometer network 121 of the same general type as control network 85and potentiometric network 84, the motor 37 under the control of anamplifier 122 adjusts the dampers 31, 32 and $3 in response to thedifference between the temperatures of the steam flow in lines 19 and25. Thus, the motor '7 at the time it adjusts contact 71a of network 84also adjusts a contact 1230 of slidewire 123. Similarly, at. the timecontact 72:: is adjusted the motor 66 adjusts contact 1241: relative toslidewire resistor 124. The differcnce in operation between the twonetworks is symbolically illustrated by the cross-over lines of network$4 as compared with network 121. For example, in the former, withcontacts 71a and 72a moving toward the left and in the direction of thelow temperature ends of the respective sl-idewires 71 and 72, it will beseen an error signal of increasing magnitude will be developed. Thiswill be made clear by arbitrarily assumed values. With the parts intheir illustrated positions, it can be assumed that between the point125 (directly connected to the negative pole of battery 81) and contact71a there is a potential rise of 2 volts. There will also be a rise inpotential between point 125 and contact 7211 of 2 volts. The differenceE between the two potentials is, of course, zero. If new contact 71a ismoved to the left toward the low temperature end of slidewire 71, thepotential rise between point 125 and contact 7111 will be decreased. Itthe potential difference at the new position be 1 volt, it will be seenthat with an adjustment of contact 72!: toward the high end of theslidewire marked H by an amount to reduce the potential rise from point125 to 1 volt, the difference, B; will again be zero. Thus, the outputor error voltage appearing between contacts 710 and 72a will remainunchanged though the temperature of the steam in line 19 decreased witha corresponding increase of temperature of the steam in line 25. In bothcases the sum of the temperatures of the steam in lines 19 and remainedthe same. Thus network 84 re sponds to change in the sum of thetemperatures of the steam in lines 19 and 25. However, if the sum of thetwo temperatures changes and in a direction to produce adjustment ofcontacts 71a and 72a in the same direction,

as for example by movement toward the high ends of the slidewires, thefollowing will illustrate the action in terms of additional assumedvalues.

If the potential rise between point 125 and the new position of contact71a be increased to 3 volts, the movement of contact 72a toward the highend will decrease the potential difference to 1 volt, and thus the errorsignal will be the difference between 3 volts and 1 volt, namely, 2volts.

In the control system 120 the motor 37 operates the follow-up element orslidewire contact 12011 for the proportional action; and the motor 88 ofsystem 85 similarly moves slidewire contact 87a.

In the network 121 movement of contact 123:: toward the left and towardthe low end of slidewire v123 will reduce the potential between point125a and contact 123a. However, movement of contact 124a toward theright and toward the high side will increase the potential difference.If the reduction in the first case reduces the potential differencebetween point 125a and contact 123a to 1 volt and the increase intemperature as detected by thermocouple 42 increases the potentialdifference between point 125a and contact 124a to 3 volts, the errorsignal will be 2 volts. Thus network 121 responds to a change in thedifference between the temperatures in lines 19 and 25.

- The network 120 includes a reset resistor 89a, reset capacitor 90a,and a rate capacitor 94a. Thus, the difference-network produces acontrol action including proportional, rate, and reset components ofcontrol. The network also includes a component developed by manometer102 by way of output winding 130, full-wave rectifier 131, slidewire132, and conductors 133 and 134 which introduces the additionalanticipatory control into control network in manner already described inconnection with network 85.

The polarity of the anticipatory voltage developed between conductors133 and 134 will be selected depending upon the particularcharacteristics of the steam generator. In some cases it will bedesirable that the anticipatory action shall produce rotation of motor37 in a direction to operate dampers 31 and 32 in passage-closingdirection, while moving the dampers 33 in a passage-opening direction.The action, in any case, will be such that with an assumed decrease inthe flow of the gases and products of combustion to the stack, theredistribution of the gas flow as between the superheater sections 17aand 17b and the reheater section 24 will maintain the difference intemperature of the steam flowing in lines 19 and 25 at the desiredpredetermined values, at least more so than if the anticipatory controlwere not included as a part of the system.

In-summary, the fluid-heating system of Fig. 1 provides for themaintenance at a selected value of the temperature of the fluid streamin line 15 at a predetermined value, while at the same time maintainingat a predetermined value the temperature of the fluid in line 25. Theforegoing is accomplished by controlling the heat exchange rates asbetween the volume or quantity of a condition-changing agent such as theheating agent com prising fine gases, recirculation of which isregulated in accordance with the sum of the temperatures of said fluidor vapor streams and the division of said agent relative to said streamsas by controlling the division of flow of the combustion gases in heatexchange with the two streams in accordance with the difference betweentheir said temperatures. The end result is the delivery at uniformtemperature of the steam supplied by way of lines 19 and 25 to the twosections of the turbine 21 which is shown driving, for example, anelectrical power generator 135.

It is to be understood that other features of control will normally beprovided in the operation of the boiler, such as a master controlresponsive to steam pressure in the steam drum, all as described inBristol Letters Patent No. 2,657,347, dated October 27, 1953, and it isto be further understood that the method of the present invention maynot only be performed manually by the electrical system of Fig. 1, butalso by other instrumentalities, such as the pneumatic systemillustrated in Fi 2, and it is to be understood the invention isapplicable to cooling systems and other conditions whose magnitudes areaffected in ways analogous to the steam temperatures above described.

In the description of the invention thus far presented, there was animplied assumption that the output voltages (between contacts 71:: and72a and between contacts 123a and 124a) of networks and 121,respectively, changed by equal amounts with equal changes in the sum ofthe temperatures detected by thermocouples 41 and and with equal changesin the difference between the temperatures detected by them. However,the assumed equality in output for the respective changes in the sumsand differences of the temperatures detected will be thepreferredoperation for those cases in which the respectivetemperaturesin lines 19 and 255 will be maintained Q by the equal adjustmentsproduced by motors 88 and 37.

In many cases the rate of heat absorption in the reheater section 24 maydiffer from the rate of heat absorption in the steam superheatersections 16, 17a and 17b, and tnus the response of the sum anddifference control networks and of the respective motors 88 and 37controlled by them should be different when the temperature detected bythermocouple 41 changes by a given amount than when the temperaturedetected by the thermocouple 42 changes by the same amount. Theadditional flexibility provided in accordance with the present inventionas embodied in the system of Fig. 1, has already been mentioned in thereference to the fact that any change in the rate of flow or quantity ofcombustion gases passing over the superheater section 16 producesoperation of the motor 37 to correct for the added or decreased heatabsorption by section 16.

The foregoing requirement is met by providing the slidewires 71 and 72with adjustable shunting resistors 75 and 76 and the slidewires 123 and124 are similarly provided with adjustable shunting resistors 75a and76a. By adjusting shunting resistor '75 to have a value differing fromshunting resistor 76 and assuming slidewires '71 and 72 to have equalresistance values, the degree of adjustment of motor 88 for a givenchange in temperature detected by thermocouple 41 may be made widelydifferent from the adjustment produced by a like change in thetemperature of the thermocouple 42. Similarly, by adjusting shuntingresistors 75a and 76a to have differing values, the degree of adjustmentof dampers 3133 produced by motor 37 may be widely different for a giventemperature change by thermocouple 41 than for a like temperature changeof the thermocouple 42.

Further flexibility is provided by varying the resistance values ofrheostats 82 and 82a to provide a different output from one of thenetworks 84 and 121 than the other for a given change in temperature andcorresponding movement of slidewire contacts 71a and 123a. For example,if the potential difference across slidewire 123 is made greater byincreasing the current flow through the adjustable resistor 82a, thefollow-up movement of the follow-up contact 120a relative to itsslidewire will be increased and will require a greater movement of motor37 to move that contact to the network balancing position. Thus, thethrottling range for the control of motor 37 may differ from thethrottling range for the control of motor 88.

Though the system has great flexibility by reason of the additionalfeatures, it is again emphasized that the network 84 continues tofunction in response to the sum of the temperatures detected bythermocouples 41 and 42 and the network 121 continues to respond to thedifference between those temperatures, and that when the sum anddifference networks have been referred to in the claims the descriptivelanguage is intended to be broad enough to include the modifyingfeatures just described.

In some applications of the invention, it may be desirable to adjust thenetwork 121 to have operating characteristics as shown by the straightlines 137 and 136R on the graph of Fig. 4 plotted with temperature asabscissae and output voltage or potential difierence as ordinates. Thus,with a change in temperature of thermocouple 41 the potential differencebetween contact 123a and point 125a of the networks will vary inaccordance with graph 13]. On the other hand, the potential betweencontact 124a and the point 125a will vary at a difierent rate and willbe represented by a line or graph of different slope as indicated by theline 136R. As previously explained, the voltage output of network 121between contacts 123a and 124a will be the difference between twovoltages just identified. With the temperatures in lines 19 and 25 atthe set points indicated on the graph of Fig. 4 respectively at T and Tthe voltage between point 125a and the contact 123:: is shown as theordinate 10 of graph 137 at T and that the voltage between point a andcontact 124a is shown as the ordinate of graph 136R at T the twovoltages being of equal magnitude. Since they are opposed as betweencontacts 123a and 124a there is zero output from network 121.

To illustrate the changed operation provided by the shunts 75a and 76afor the slidewires, it will now be assumed that the temperature ofthermocouple 42 is increased to the value T and that the temperature ofthermocouple 41 is increased to the value T While the temperature ofthermocouple 41 has increased to greater degree than the temperature ofthermocouple 42 with an increase in their difference between them,nevertheless, the network 121 has zero output by reason of the fact thatthe change of voltage per degree change of temperature is greater forthe thermocouple 42, graph 136R, than it is for the thermocouple 41,graph 137. Thus, the operation of the difference network 121 has beenmodified so that it produces an output voltage when the differencebetween the steam temperatures in lines 19 and 25 attains valuesrequiring damper adjustments; within the limits established by aparticular installation adjustments will not be needed sincecompensating factors resulting from the particular heatingcharacteristics of the superheater sections and the heatingcharacteristics of the reheater section themselves tend to keep thesteam temperatures at the control or set points.

The foregoing flexibility likewise applies to the sum control network 84where the graph 136R of Fig. 4 represents changes in potentialdifference between the point 125 and the contact 72a with temperaturechange, while the graph 1378 represents changes in potential differencebetween point 125 and slidewire contact 710 with change in temperatureof thermocouple 41. At the temperature T for thermocouple 41 and thetemperature T for the thermocouple 42, it will be obvious by referenceto graphs 136R and 1378 that the voltage outputs are equal. Because theyare equal and are opposed as between contacts 71a and 72a, the output ofnetwork 84 is zero and the two temperatures are at the set points asbefore. If it now be assumed that the temperature of thermocouple 42 isincreased to T and that of thermocouple 41 has decreased to T411 and bythe same amount as the increase, the sum of the two will be changed overits previous value but the voltage, graph 136R, at T has the same valueas the voltage indicated on graph 1378 at the temperature T The responseof the network 84 has been somewhat modified to take into account thevariable heating characteristics of the reheater and superheatersections. Nevertheless, it continues to function whenever the sumdiffers by an amount which is not compensated for by the inherentcharacteristics of a particular heating system. The graphs of Fig. 4 areto be taken as illustrative of the flexibility of the system and themanner in which it may be readily adapted to a wide variety ofapplications including steam generators of greatly differing design.

Referring to Fig. 2, the invention has been shown as applied tobalanceable systems of quite a different type but to a fluid heatingsystem the same as in Fig. 1. In fact certain details of the steamgenerator of Figs. 1 and 2 have already been described. In Fig. 2, thesecondary superheater is diagrammatically shown at 16 with similardiagrammatic showing of one primary superheater 17a and the reheatercoil 24. Thermocouples 41 and 42, in association, with measuring systems140 and 141 of the same type as illustrated and described in theembodiment of Fig. 1, indicate on their respective scales and charts140a, 1411b and 141a, 141b, the respective temperatures of the steamflow in lines 19 and 25. The motor 57 of network 140 operable inaccordance with the temperature in line 19 adjusts a control screw 142to change the position of a baffle 143 and thus varies the air or fluidpressure developed by a booster 144 in a line 145. For a detaileddisclosure of the operation of the illustrated pneumatic system,reference is to be had to McLeod Patent No. 2,507,606, the booster 144corresponding with the booster 56 of that patent, and the pressure inline corresponding with the pressure in line 68 of said patent. Insteadof the reset bellows 46 of said patent, there is shown in Fig. 2 aspring 146, which is preferably adjustable. While the system may includea reset control component and a rate component in manner di'sclosed insaid patent, the spring 146 has been provided for a system with onlyproportional action present.

As explained in said McLeod patent, the pressure in line will vary inaccordance with the change in the temperature detected by thermocouple41. In a similar manner, the motor 66, whose energization is'under thecontrol of the potentiometer network 141 and amplifier 65, will adjustthe-control screw 142a to position the baffie 143a relative to theassociated nozzle. Through the action of the booster 144a the pressurein line 147 will vary with change in the temperature detected bythermocouple 42.

The pressure in line 147 is transmitted by way of line 148 to a bellows149 acting on a pivoted lever 150'in a direction tending to move itcounterclockwiseagainst the opposing pressure of a spring 151. A bellows152 receiving the pressure from line 145 tends to rotate the lever 156in the same counterclockwise direction to change the position of thatlever relative to nozzle 153 which, through a booster 1441;, establishesa pressure in line 154 which varies in accordance with the sum of thepressures in lines 147 and 145. Thus, the pressure in line 154 will varyas the sum of the temperatures of the steam flow in lines 19 and 25. Afollow-up bellows 155 responsive to the output pressure in line 154tends to rotate the lever 150 in a clockwise direction and providesproportionality between the pressure 154 and the sum of the pressures inlines 145 and 148.

The pressure in line 154 is applied to a pneumatic operator 156 whichmay be of the same type as diagrammatically illustrated in said McLeodpatent for the oper-- ation of the valve identified in that patent bythe reference character 11. The pneumatic actuator 156 positions thedamper 45 in the inlet pipe to the recirculating fan 47.

It will be observed that the pressures in lines 145 and 147 arerespectively applied to bellows 157 and 158, which with respect to alever 159 develop forces tending to rotate it in opposite directions.Thus, the position of pivot lever 159 relative to a nozzle 160 willdepend upon the difference in the magnitude of the two pressures appliedto bellows 157 and 158. The spring 161 opposes a proportional bellows162, and a booster 144c establishes in a line 162a a pressureproportional to the difference between the pressures applied to bellows157 and 158. The pressure in line 162a is applied to a pneumaticactuator 163 of the same type as actuator 156' and operates to positionthe lever 34 to position the dampers 31, 32 and 33 of Fig. 1, only thedampers 32 being shown in Fig. 2.

From the foregoing description it will be seen that apparatus has beenprovided for carrying out the invention both by the balanceable networksor electrical systems of Fig. 1 and by the balanceable systems of'Fig. 2shown as of the pneumatic type. In accordance with both systems themagnitudes of two conditions are controlled and maintained to selectedvalues by varying the setting of two condition-varying elements whereone of the latter affects both of the conditions in the same sense andwhere another of them afiects them in opposite sense.

Variation in the operation of the sum and difference controllers, of thepneumatic system of Fig. 2 maybe desired for the same reasons discussedinconnection with the slidewire shunting resistors of Fig. 1. Asexplained-in saidMcLeod patent, the nozzles associated with bafiies-143and 143a-are adjustable to vary the throttlingranges. By makingthesprings 146, 151, 161

12 and 165 adjustable, further flexibility is provided. Similarly theextent of movement of each control screw, 142, 142a, for'a given changein temperature may be made to differ by providing different gear ordriving ratios between them and their respective motors 57 and 66.

It is again emphasized that while the invention has been shown asparticularly applicable to temperature control systems, it is not to belimited thereto, the scope of the invention being commensurate with thatof the appended claims.

What is claimed is:

1. A control system for maintaining the magnitudes of two conditions atpredetermined values by maintaining both the magnitude of their sum andthe magnitude of their difference at predetermined values, comprisingmeans including two condition-varying elements a first of which isoperable concurrently to vary the magnitudes of said conditions inopposite senses and the second of which is operable concurrently to varythe magnitudes of said conditions in the same sense, first and secondcondition-sensitive means respectively responsive to the magnitudes ofsaid two conditions, a balanceable summing system which is balanced whensaid sum is at a predetermined value, a balanceable difference systemwhich is balanced when said difference is at a predetermined value, eachof said balanceable systems having a first adjustable element and asecond adjustable element respectively affecting the balance of saidsystems, means responsive to said first condition-sensitive means forconcurrently actuating said first adjustable element in each of saidsystems, means responsive to said second condition-sensitive means foractuating said second adjustable element in said summing system forproducing in conjunction with said adjustment of said first element anunbalance of said summing system upon deviation of the sum of themagnitudes of said conditions from said predetermined value and forconcurrently actuating said second adjustable element in said differencesystem for producing in conjunction with said first element in saiddifference system unbalance of said difference system upon deviation ofthe difference between said two conditions from said predeterminedvalue, each of said systems including an adjustable system-balancingelement, a first actuating means responsive to the unbalance of saidsumming system in one direction for actuating one of saidcondition-varying elements to change in the same direction themagnitudes of said conditions for changing the sum of said magnitudes toapproach said predetermined value of said sum, a second actuating meansresponsive to the unbalance of said difference sys-' tem forconcurrently actuating the other of said condition-varying elements forconcurrently changing the magnitudes of said conditions in oppositedirections to change the difference between said magnitudes in adirection to approach said predetermined value of said difference, eachof said actuating means having a connection for adjustment respectivelyof said system-balancing elements in said systems in directions torestore balance of each of them.

2. A control system for maintaining the magnitudes of two conditions atpredetermined values by maintaining both the magnitude of their sum andthe magnitude of their difference at predetermined values, comprisingmeans including two condition-varying elements, a first of which isoperable concurrently to vary the magnitudes of said conditions inopposite senses and the second of which is operable concurrently to varythe magnitudes of said conditions in the same sense, first and secondcondition-sensitive means respectively responsive to the magnitudes ofsaid two conditions, a balanceable electrical summing network which isbalanced when said sum is at a predetermined value, a balanceableelectrical difference network which is balanced when said difference isat a predetermined value, each of said balanceable networks having afirst adjustable impedance element and a second adjustable impedanceelement respectively affecting the balance of said networks, meansresponsive to said first condition-sensitive means for concurrentlyadjusting said first adjustable impedance element in each of saidnetworks, means responsive to said second condition-sensitive means foradjusting said second adjustable impedance element in said summingnetwork for producing in conjunction with said adjustment of said firstimpedance element an unbalance of said summing network upon deviation ofthe sum of the magnitudes of said conditions from said predeterminedvalue and for concurrently adjusting said second adjustable impedanceelement in said difference network for producing in conjunction withsaid first impedance element in said difference network unbalance ofthat network upon deviation of the difference between said twoconditions from said predetermined value, each of said networks alsoincluding an adjustable network-balancing impedance element, a firstactuating means responsive to the unbalance of said summing network inone direction for actuating one of said condition-varying elements tochange in the same direction the magnitudes of said conditions forchanging the sum of said magnitudes to approach said predetermined valueof said sum, a second actuating means responsive to the unbalance ofsaid difference network for concurrently actuating the other of saidcondition-varying elements for concurrently changing the magnitudes ofsaid conditions in opposite directions to change the difference betweensaid magnitudes in a direction to approach said predetermined value ofsaid difference, each of said actuating means having a connection forrespectively adjusting said system-balancing impedances in said networksin directions to restore balance of each of them.

3. In a fluid heating system having separate flow passages for a heatingagent, fluid-conducting heating sections disposed in each of saidpassages for maintaining the fluid outlet temperatures of the respectivesections at predetermined values, the combination of two adjustableflowcontrolling elements for said agent, a first of said flowcontrollingelements being adjustable to change the quantity of said heating agentpassing through said separate flow passages to change in the samedirection said fluid outlet temperatures, and the second of saidflow-controlling elements having structure disposed in reference to saidpassages adjustable for varying the division of said quantity of saidheating agent between said sections to change said fluid outlettemperatures in opposite directions, a first control means including abalanceable electrical network with impedances respectively adjustablein response to said fluid outlet temperatures in direction and to extentwhich unbalances said network in accordance with the sum of said outlettemperatures for operating said first flow-controlling element forchanging the quantity of said agent in a direction and by an amount tomaintain said sum at a predetermined value, a second control meansincluding a second balanceable network having impedances respectivelyadjustable in response to said fluid outlet temperatures in directionand extent which unbalances said network in accordance with thediflerence between said outlet temperatures for operating, concurrentlywith adjustment of said first condition-varying element, said secondcondition-varying element in a direction and by an amount to vary thedivision of said agent between said flow passages for maintaining at apredetermined value said difference between said outlet temperatures,whereby said fluid outlet temperatures are maintained at saidpredetermined values, and means responsive to changes in the quantity ofsaid heating agent which occur independently of the effect of said firstflow-controlling element for adjusting said first flow-controllingelement in a direction to anticipate and to compensate for said chan ein said quantity.

" nuid heating system having separate flow passages for a heating agent,fluid-conducting heating sections disposed in each of said passages formaintaining the fluid outlet temperatures of the respective sections atpredetermined values, the combination of two adjustable flowcontrollingelements for said agent, a first of said flowcontrolling elements beingadjustable to change the quantity of said heating agent passing throughsaid separate flow passages to change in the same direction said fluidoutlet temperatures, and the second of said flow-controlling elementshaving structure disposed in reference to said passages adjustable forvarying the division of said quantity of said heating agent between saidsections to change said fluid outlet temperatures in oppositedirections, a first control means including a balanceable electricalnetwork with impedances respectively adjustable in response to saidfluid outlet temperatures in direction and to ext nt which unbalancessaid network in accordance with the sum of said outlet temperatures foroperating said first flow-controlling element for changing the quantityof said agent in a direction and by an amount to maintain said sum at apredetermined value, a second control means including a secondbalanceable network having impedances respectively adjustable inresponse to said fluid outlet temperatures in direction and extent whichunbalances said network in accordance with the difference between saidoutlet temperatures for operating, concurrently with adjustment of saidfirst condition-varying element, said second condition-varying elementin a direction and by an amount to vary the division of said agentbetween said flow passages for maintaining at a predetermined value saiddiflerence between said outlet temperatures, whereby said fluid outlettemperatures are maintained at said predetermined values, and meansresponsive to changes in the quantity of said heating agent which occurindependently of the effect of said first flowcontrolling element forapplying to one of said networks a network unbalancing voltage foradjusting one of said flow-controlling elements independently of saidpredetermined values of said sum and of said diflerence.

5. In a fluid heating system having separate flow passages for a heatingagent, fluid-conducting heating sections disposed in each of saidpassages for maintaining the fluid outlet temperatures of the respectivesections at predetermined values, the combination of two adjustableflowcontrolling elements for said agent, a first of said flowcontrollingelements being adjustable to change the quantity of said heating agentpassing through said separate flow passages to change in the samedirection said fluid outlet temperatures, and the second of saidflow-controlling elements having structure disposed in reference to saidpassages adjustable for varying the division of said quantity of saidheating agent between said sections to change said fluid outlettemperatures in opposite directions, a first control means including abalanceable electrical network with impedances respectively adjustablein response to said fluid outlet temperatures in direction and to extentwhich unbalances said network in accordance with the sum of said outlettemperatures for operating said first flow-controlling element forchanging the quantity of said agent in a direction and by an amount tomaintain said sum at a predetermined value, a second control meansincluding a second balanceable network having impedances respectivelyadjustable in response to said fluid outlet temperatures in directionand extent which unbalances said network in accordance with thedifference between said outlet temperatures for operating, concurrentlywith adjustment of said first condition-varying element, said secondcondition-varying element in a direction and by an amount to vary thedivision of said agent between said flow passages for maintaining at apredetermined value said difierence between said outlet temperatures,whereby said fluid outlet temperatures are maintained at saidpredetermined values, and means responsive to changes in the quantity ofsaid heating agent which occur independently or the effect of said firstflow-controlling element for applying to both of said networksunbalancing voltages for adjustment of both of said flowcontrollingelements independently of said predetermined values of said sum and ofsaid diiference.

6. In a fluid heating system having separate flow passages for a heatingagent, fluid-conducting heating sections disposed in each of saidpassages for maintaining the fluid outlet temperatures of the respectivesections at predetermined values, the combination of two adjustablefiow-controlling elements for said agent, a first of saidflow-controlling elements being adjustable to change the quantity ofsaid heating agent passing through said separate flow passages to changein the same direction said fluid outlet temperatures, and the second ofsaid flow-controlling elements having structure disposed in ref erenceto said passages adjustable for varying the division of said quantity ofsaid heating agent between said sections to change said fluid outlettemperatures in opposite directions, a first control means including abalanceable electrical network with impedances respectively adjustablein response to said fluid outlet temperatures'in direction and to extentwhich unbalances said network in accordance with the sum of said outlettemperatures for operating said first flow-controlling element forchanging the quantity of said agent in a direction and by an amount tomaintain said sum at a predetermined value, a second control meansincluding a second balanceable network having impedances respectivelyadjustable in response to said fluid outlet temperatures in directionand extent which unbalances said network in accordance with thedifference between said outlet temperatures for operating, concurrentlywith adjustment of said first conditionvarying element, said secondconditionvarying element in a direction and by an amount to vary thedivision of said agent between said flow passages for maintaining at apredetermined value said difference between said outlet temperatures,whereby said fluid outlet temperatures are maintained at saidpredetermined values, and means responsive to changes in acharacteristic of said heating agent which occur independently of theeifect of either of said flow-controlling elements for applying to bothof said networks unbalancing voltages for adjustment of both of saidflow-controlling elements independently of said predetermined values ofsaid sum and of said ditference.

7. In a fluid heating system having separate flow passages for a heatingagent, fluid-conducting heating sections disposed in each of saidpassages for maintaining the fluid outlet temperatures of the respectivesections at predetermined values, the combination of two adjust ableflow-controlling elements for said agent, a first of saidflow-controlling elements being adjustable to change the quantity ofsaid heating agent passing through said separate flow passages to changein the same direction said fluid outlet temperatures, and the second ofsaid flow-controlling elements having structure disposed in reference tosaid passages adjustable for varying the division of said quantity ofsaid heating agent between said sections to change said fluid outlettemperatures in opposite directions, a first control means including abalanceable electrical network with impedances respectively adjustablein response to said fluid outlet temperatures in direction and to extentwhich unbalances said network in accordance with the sum of said outlettemperatures for operating said first flow-controlling element forchanging the quantity of said agent in a direction and by an amount tomaintain said sum at a predeter mined value, and a second control meansincluding a second balanceable network having impedances respectivelyadjustable in response to said fluid outlet temperatures in directionand extent which unbalances said network in accordance withthedifference between said outlet temperatures for operating, concurrentlywith adjustment of said first condition-varying element, said secondconditionvarying element in a direction and by an amount to vary thedivision of said agent between said flow passages for maintaining at apredetermined value said difference between said outlet temperatures,whereby said fiuid outlet temperatures are maintained at saidpredetermined values.

8. In a steam generator having separate flow passages for hot productsof combustion, a steam superhcatcr section and, a steam reheater sectionrespectively disposed in said passages and in heat exchange relationwith said hot products of combustion and having adjustable control meansfor concurrently changing the heat input from said hot products ofcombustion to said sections and damper means for varying the division ofthe hot products of combustion between said sections, the combination ofmeans for maintaining at predetermined values the respective exittemperatures of steam passing through said sections by maintaining themagnitude of their sum at a predetermined value and by maintaining themagnitude of their difference at a predetermined value, comprisingfirstand second temperature-sensitive means responsive to the exittemperatures of the steam from said sec tions, a balanceable summingsystem, a balanceable difference systemfeach of said balanceable systemshaving a first adjustable element and a second adjustable elementrespectively affecting the balance of said systems, means responsive tosaid first temperature-sensitive means for concurrently actuating saidfirst adjustable element in each of said systems, means responsive tosaid second temperature-sensitive means for actuating said secondadjustable element in said summing system for producing in conjunctionwith said adjustment of said first element an unbalance of said summingsystem upon deviation of the sum of the magnitudes of said conditionsfrom said predetermined value and for concurrently actuating said secondadjustable element in said difference system for producing inconjunction with said first element in said ditference system unbalanceof said difference system upon deviation of the difference between saidtwo conditions from said predetermined value, each of said systemsincluding an adjustable system-balancing element, a first actuatingmeans responsive to the unbalance of said suinming system in onedirection for actuating said control means concurrently to increase theheat input to both of said sections and upon unbalance of said summingsystem in the opposite direction for actuating said control meansconcurrently to decrease said heat input to said sections, saidactuating means having a driving connection for concurrently adjustingsaid system-balancing element of said summing system in a direction torestore balance of said summing system, and a second actuating meansresponsive to the unbalance of said difference system in one directionfor actuating said damper means concurrently to increase the heat inputto one of said sec tions and to decrease the heat input to the other ofsaid sections and operable in response to unbalance of said differencesystem in the opposite direction for decreasing the heat input to saidone of said sections while increasing the heat input to the other ofsaid sections, said damper means being actuated in a direction whichrelatively changes the heat input to said sections in a direction toreduce departure of said difference between said steam temperatures fromits said predetermined value, said second actuating means having adriving connection for concurrently adjusting said system-balancingelement of said difference system in a direction to restore balance tosaid diiference system, whereby upon deviation of either or both of saidsteam temperatures from their predetermined values the deviating steamtemperature is returned to its said predetermined value with a minimumchange in the other steam temperature during the concurrentreadjustments of said heat inputs to said sections by both saidactuating means.

9. In a steam generator having separate flow passages for hot productsof combustion, a steam superheater section and a steam reheater sectionrespectively disposed in said passages and in heat-exchange relationwith said hot products of combustion, change in the total quantity ofcombustion products which flows through said passages varying the heatinputs to said sections in the same direction and change in the divisionof flow of said products of combustion through said passages increasingthe heat input to one of them while decreasing the heat input to theother of them, means for recirculating combustion gases to vary saidquantity of said products of combustion flowing through said passagesand damper means disposed in relation to said passages for varying saiddivision between them of said products of combustion, the combination ofa balanceable summing network and a balanceable difference network, eachsaid network having a pair of adjustable circuit elements and eachhaving a follow-up adjustable circuit element, means respectivelyresponsive to the outlet steam temperatures of said superheater sectionand of said reheater section for respectively and concurrently adjustingsaid pairs of adjustable circuit elements for unbalancing said pair ofnetworks, said pair of said circuit elements in said summing networkbeing connected for unbalancing it in response to change in the sum ofthe magnitudes of said steam temperatures and the other of said pair ofadjustable circuit elements being connected in the other of saidnetworks for unbalancing it in response to change in the differencebetween the magnitures of said' steam temperatures, first control meansresponsive to the unbalance of said summing network for controlling saidrecirculating means for increasing said quantity of said products ofcombustion flowing through said passages upon change in the unbalance ofsaid summing network in one direction and for decreasing said quantityof said products of combustion upon change in the unbalance of saidsumming network in the opposite direction, said first control meansbeing connected to adjust said follow-up circuit elements in saidsumming network in a direction to restore balance of said summingnetwork, and a second control means responsive to the unbalance in onedirection of said difference network for operating said damper means inone direction and responsive to the unbalance of said network in anopposite direction for operating said damper means in an oppositedirection for varying said division between said passages of saidproducts of combustion to increase the heat transfer to one of saidsections and to decrease the heat transfer to the other of saidsections, said second control means being connected to adjust saidfollow-up element in said ditference network in a direction to restorebalance to said difference network.

References Cited in the file of this patent UNITED STATES PATENTS2,298,700 Junkins et a1. Oct. 13, 1942 2,575,885 Mittendorf Nov. 20,1951 2,579,027 Walter et al Dec. 18, 1951 2,590,712 Lacerenza Mar. 25,1952 2,649,079 Van Brunt Aug. 18, 1953 2,653,447 Heller Sept. 29, 19532,685,279 Caracristi Aug. 3, 1954

